Fault current limiter with a plurality of superconductiong elements, at least one of which with an electric contact between its superconducting film and its electrically conducting substrate

ABSTRACT

A fault current limiter, with a superconducting device ( 1; 21; 31; 41; 51; 61; 71; 72 ) comprising a sequence of superconducting elements ( 2   a - 2   f ), each with an electrically conducting substrate ( 3   a - 3   d ), a superconducting film ( 5   a - 5   d ), and an electrically insulating intermediate layer ( 4   a - 4   c ) provided between the substrate and the superconducting film, wherein the superconducting films ( 5   a - 5   d ) of adjacent superconducting elements ( 2   a - 2   f ) of the sequence are electrically connected, in particular in series, wherein the electrically conducting substrate ( 3   a - 3   d ) of each superconducting element ( 2   a - 2   f ) of the sequence is electrically insulated from each electrically conducting substrate ( 3   a - 3   d ) of those adjacent superconducting elements ( 2   a - 2   f ) within the sequence whose superconducting films ( 5   a - 5   d ) are electrically connected in series with the superconducting film ( 5   a - 5   d ) of said superconducting element ( 2   a - 2   f ). At least one of the superconducting elements ( 2   a - 2   f ) comprises an electric contact ( 24   a - 24   c ) between its superconducting film ( 5   a - 5   d ) and its electrically conducting substrate ( 3   a - 3   d ) through its insulating layer ( 4   a - 4   c ), wherein the electrical contact ( 24   a - 24   c ) is located basically in the middle between the regions where the superconducting element ( 2   a - 2   f ) is electrically connected to a previous and a next superconducting element ( 2   a - 2   f ).

This application is a divisional of Ser. No. 12/998,714, which is thenational stage of PCT/EP2009/008366 filed Nov. 25, 2009 and claimingParis convention priority of EP 08020789.7 filed Nov. 29, 2008, theentire disclosures of which are hereby incorporated by reference

BACKGROUND OF THE INVENTION

The invention relates to a fault current limiter, with a superconductingdevice comprising a sequence of superconducting elements, each with

-   -   an electrically conducting substrate,    -   a superconducting film, and    -   an electrically insulating intermediate layer provided between        the substrate and the superconducting film,        wherein the superconducting films of adjacent superconducting        elements of the sequence are electrically connected, in        particular in series,        wherein the electrically conducting substrate of each        superconducting element of the sequence is electrically        insulated from each electrically conducting substrate of those        adjacent superconducting elements within the sequence whose        superconducting films are electrically connected in series with        the superconducting film of said superconducting element.

Such a superconducting device is known from U.S. Pat. No. 7,071,148 B1.

Superconducting fault current limiters are used to limit the currentflow through the load side of an electric circuitry in case of a shortcircuit on the load side. In the most simple case, the fault currentlimiter comprises a superconducting device connected in series with theload. The superconducting device can carry a current with very low loss.As long as the current through the superconducting device does notexceed the critical current, the superconducting device is practicallyinvisible within the electric circuitry, and it is the characteristics,in particular the resistance, of the load which determine the currentwithin the electric circuitry.

In case the resistance of the load drops (i.e. there is a short circuitwithin the load), the current in the circuitry increases, and eventuallyexceeds the critical current. In this case, the superconducting devicequenches (i.e. becomes normally conductive) which results in a highohmic resistance of the superconducting device. As a result, the currentin the electric circuitry drops accordingly, and the load is protectedfrom high electric current.

The superconductor material of a superconducting device must be cooledto a low temperature in order to attain its superconducting state. Inorder to facilitate and lessen the costs for the cooling, hightemperature superconductor materials (HTS materials) may be used. HTSmaterials have a critical temperature above a temperature of 30 K, andcan often be cooled with liquid nitrogen (LN2).

A fault current limiter with a superconducting device using a HTSmaterial is known from U.S. Pat. No. 5,986,536. It comprises severalsuperconducting elements, each comprising a HTS film deposited on anelectrically insulating (dielectric) substrate, in particular made of amaterial that permits textured growth of the HTS film such asyttrium-stabilized ZrO₂, with a thin interlayer of Ag. Thesuperconducting elements, i.e. their HTS films, are connected in series.

Fault current limiters of this type comprising superconducting elementswith textured superconducting films deposited on a dielectric substrate,are rather expensive to produce. Further, such fault current limitershave a relatively long recovery time after a quench event.

DE 2410148 A discloses a switching device comprising a plurality ofaluminium plates punched out in a meander type fashion, with the platescoated with superconducting layers and having aluminium oxide layers inbetween. The superconducting layers of neighbouring plates are connectedin series via protruding contacts.

U.S. Pat. No. 7,071,148 B1 describes a joined superconducting article,wherein two neighbouring and similarly oriented segments, eachcomprising a metal based substrate, a buffer layer and a superconductivecoating, are electrically connected via a third, opposing such segmentarranged on top of the two segments.

It is the object of the invention to introduce a fault current limiterwhich is cost efficient in production, and which is capable of a shortrecovery time after a quench event.

SUMMARY OF THE INVENTION

This object is achieved, in accordance with the invention, by a faultcurrent limiter as introduced in the beginning, characterized in that atleast one of the superconducting elements comprises an electric contactbetween its superconducting film and its electrically conductingsubstrate through its insulating layer, wherein the electrical contactis located basically in the middle between the regions where thesuperconducting element is electrically connected to a previous and anext superconducting element.

According to the invention, the fault current limiter (=FCL) isbasically superconducting device comprising a plurality ofsuperconducting elements; these superconducting elements each have asuperconducting film, in particular a HTS film, deposited on anelectrically conducting substrate, in particular a metal substrate. Thesuperconducting film and the electrically conducting substrate are (atleast to a very large extend) insulated from each other; for thispurpose there is an insulating intermediate layer deposited between theelectrically conducting substrate and the superconducting film. It isnoted that superconducting wires based on electrically conductingsubstrates with a HTS material deposited on top are known e.g. from U.S.Pat. No. 6,765,151.

A superconducting element based on an electrically conducting substrateis much more cost efficient in production than a superconducting elementbased on a dielectric substrate, in particular due to the substratecosts being different by a factor of about 50. Further, an electricallyconducting substrate, typically a metal substrate, offers also a muchbetter heat conduction than typical dielectric substrates. As aconsequence, after a quench event which typically heats thesuperconducting element above the critical temperature of the materialof the superconducting film, it can be cooled much faster so the faultcurrent limiter recovers normal operation much more rapidly.

As a particularity of the invention, the inventive FCL applies asequence of superconducting elements, with their superconducting filmselectrically connected, and with their electrically conductingsubstrates insulated from each other. This inventive design has beenfound beneficial for achieving both a high fault resistance, and asufficiently high possible voltage drop (electrical field) across thedirection of current transport, in particular ≧2 V/cm. Allowing toachieve a high fault resistance (i.e. resistance of the fault currentlimiter in the quenched mode), and allowing to achieve a high voltagedrop, can therefore be considered as further objectives of the presentinvention.

When the inventive FCL is in the fault modus, i.e. when the load has ashort circuit and the superconducting films have quenched, the externalvoltage drops across the fault current limiter, or more exactly acrossthe now resistive (normal-conducting) superconducting films within thefault current limiter. However, there is a bypass current path, offeredby the electrically conducting substrates of the superconductingelements nearby. If the voltage is high enough, it may cause a voltagebreakthrough from a superconducting film through an insulatingintermediate layer to an electrically conducting substrate.

If a bypass current path through the substrates became active, the faultresistance of the fault current limiter would drop significantly, andthe fault current through the FCL would increase, endangering the loadto be protected. Moreover, the ohmic heating of the FCL in the faultcase would increase, thus prolonging the time required to cool the FCLbelow the critical temperature of its superconducting films again. Theheat dissipation itself can also damage the superconductor material inthe FCL, thus limiting the maximum voltage drop across the direction ofcurrent transport.

By means of the invention, the electrically conducting substrates ofadjacent (neighbouring) superconducting elements connected in series areinsulated from each other. As a result, only a part of the externalvoltage drops across the length of an electrically conducting substrate,basically corresponding to its fraction of the overall length of thesuperconducting films connected in series in the FCL. So by means of theinventive separation of the electrically conducting substrates, thevoltage drop across the length of an electrically conducting substratecan be adjusted, and in particular reduced to a value that safelyexcludes a voltage breakthrough through the insulating layer. Inaccordance with the invention, nowhere in the superconducting devicethere is a potential difference of the total external voltage across theinsulating layer, but only of a fraction of the total external voltage,which is not high enough to cause a breakthrough; then a bypass currentpath through the electrically conducting substrate remains inactive.

In accordance with the invention, at least one of the superconductingelements comprises an electric contact between its superconducting filmand its electrically conducting substrate through its intermediate(insulating) layer, wherein the electrical contact is located basicallyin the middle between the regions where the superconducting element iselectrically connected to a previous and a next superconducting element.By this means, the voltage drop across the insulating intermediate layercan be further reduced, namely basically halved. It is noted that theelectrical contact should, in the direction of the current flow, onlyextend over a small distance (as compared to the length of thesuperconducting element), in order not to offer a bypass current pathover a significant length which would reduce the fault resistance.

Thus in the inventive FCL, whilst in fault mode, the resistance of thesuperconductor can be kept high and consequently, the Joel's heatingkept low. The latter reduces the recovery time of the FCL after a quenchevent, and allows a higher voltage drop across the superconductor filmswithout risk of damaging the superconductor material.

It is noted that preferably, in accordance with the invention, theelectrically conducting substrate of each superconducting element of thesequence is electrically insulated from each electrically conductingsubstrate of all superconducting elements within the sequence whosesuperconducting films are electrically connected in series with thesuperconducting film of said superconducting element.

In accordance with the invention, the electrically conducting substrateis typically metallic, in particular in the form of a thin tape with athickness ranging typically between 5 μm and 100 μm. The thin tape mayfacilitate the cooling of superconducting films, since they have a lowheat capacity and have good heat conductivity through the substrate, inparticular allowing a highly efficient double-sided cooling ofsuperconducting elements from the top side and through the substrateside.

Electrically connected superconducting films or adjacent superconductingelements are typically connected by a jointing means, wherein thesuperconducting films have surface area parts which are not covered witha jointing means (“free surface part”). The free surface part may range,in particular, from 10% to 98% of the overall film surface, andtypically makes the vast majority of the overall film surface.

It is noted that a superconducting device of an inventive fault currentlimiter may be supplemented by further superconducting elements of othertypes than described above; however the latter are not further referredto.

The inventive fault current limiter can be used in an electric circuitrycomprising a voltage source, in particular power plant or a power supplynetwork, connected in series with a load, in particular a transformersubstation, and connected in series with the inventive fault currentlimiter. The invention is particularly suited for high voltage sources,with voltages of 1000 V and above, in particular 10 kV and above.

In a preferred embodiment of the inventive fault current limiter, thesuperconducting films of at least some adjacent superconducting elementsare directly electrically connected. A direct electrical connection issimple to realise. Directly electrically connected means in particularthat no intermediate superconducting section is involved. Superconductorelements with directly electrically connected superconducting films arefurther referred to as directly electrically connected superconductorelements.

In a preferred further development of this embodiment, the directlyelectrically connected adjacent superconducting elements are

-   -   oriented with their superconducting films facing each other, and    -   displaced against each other, such that the adjacent        superconducting elements partially overlap in an overlap region,        wherein in the overlap region, the superconducting films of the        overlapping superconducting elements are electrically connected.        This arrangement is simple to produce.

Another preferred further development is characterized in that thesuperconducting films of the directly electrically connected adjacentsuperconducting elements are electrically connected through a layer of anormally conducting metal. Interconnecting the superconducting films inthis way is both simple and highly reliable due to a relatively largecontact area, involving only a minimum of interfaces. Typically, theconnecting layer exhibits a multilayer structure which comprisetransient sub-layers and a central layer.

The sub-layers are formed either directly on the surface of thesuperconducting film or on the surface of the superconducting filmalready coated with very thin protection layer of metal. Their task isto provide a low interfacial resistance regarding the superconductingfilm as well as to ensure a stable mechanical bonding.

These sub-layers are typically made of a precious metals or an alloy ofprecious metals, in particular comprising gold and/or silver.Alternatively, the sublayers may be based on copper or copper basedalloys as Cu—Ag, Cu—Ag—In. Furthermore, each sub-layer may compriseseveral layers as e.g. a “ground” layer of a precious metal, and asecond layer of copper (deposited for example by galvanic plating).Additionally, these several layers may comprise also a final layer madee.g. of Ag or Au or metallic alloys. The task of the final layer is toprovide passivation of the surface against chemical reactions in orderto improve quality of soldering, and thus to achieve low (e.g. <10⁻⁷Ohms×cm²) interfacial resistance within the electrical connection.

The central layer may comprise a solder which allows to provide acost-efficient connection of the superconducting films, alternatively ofthe sub-layers deposited onto these films. Typically the solder is alow-melting temperature metal, e.g. In, Zn, Cd, Ga, Bi, Ag or alloysbased on such metals. Alternatively, the central layer may be formed asa diffusion layer provided via either cold welding (under pressure) orthermal diffusion (at e.g. 400° C. in case of Ag sub-layers).

Further preferred is an embodiment wherein the superconducting films ofat least some adjacent superconducting elements are electricallyconnected by means of a bridge element,

wherein the bridge element comprises a superconducting section, andwherein the superconducting films of the adjacent superconductingelements electrically connected by means of a bridge element are bothelectrically connected to the superconducting section. The bridgeelement and its superconducting section allow more freedom ininterconnecting the superconducting films, in particular when theadjacent (neighbouring) superconducting elements with superconductingfilms connected in series are separated by a significant distance. Theresistance between the connected superconducting films during normaloperation can thus be kept low. Superconducting elements withsuperconducting films electrically connected by means of a bridgeelement are further referred to as superconducting elements electricallyconnected by means of a bridge element.

In an advantageous further development of this embodiment, the adjacentsuperconducting elements electrically connected by means of a bridgeelement are

-   -   oriented with their superconducting films facing in the same        direction,    -   and arranged next to each other, with a gap between two adjacent        superconducting elements,        wherein the bridge element establishes an electrical connection        of the superconducting films of the adjacent superconducting        elements across the gap. The gap is a simple way for mutual        insulation.

Another preferred further development of the above embodiment ischaracterized in that the bridge element comprises a dielectricsubstrate, and the superconducting section is a superconducting layercovering the dielectric substrate,

that the superconducting layer of the bridge element faces thesuperconducting films of the adjacent superconducting elementselectrically connected by means of the bridge element,

and that the bridge element overlaps, in particular partially overlaps,with both adjacent superconducting elements electrically connected bymeans of the bridge element. The connections are simple and highlyreliable due to a large contact area, with only a minimum of interfacesestablished.

In a preferred embodiment of the inventive fault current limiter, thesuperconducting elements are connected in a ring shaped fashion. Such aring shaped connection has, in case of circular currents, to beconsidered as a series sequence of the superconducting elements. Inparticular, only two superconducting elements can be connected in a ringshaped fashion. Preferably, the substrates resp. the superconductingelements as a whole are bent to give a basically circular ring-shapedarrangement. Ring shaped (short-circuited) superconducting devices areused in inductive (transformer-based) fault current limiters, whereinthe load is connected in series to the primary side, and the ring-shapedsuperconducting device is attached at the secondary side of thetransformer to shield the secondary side in the normal modus. Theinductive FCLs are particularly suitable for limiting AC currents.

In another preferred embodiment, the superconducting elements areconnected in a linear sequence. This embodiment is particularly suitablefor limiting DC currents, in particular wherein a high external voltageis divided among the superconducting elements of the linear sequence.

In a particularly preferred embodiment, the fault current limiter ischaracterized in that the sequence of superconducting elements comprisesat least three superconducting elements. In this case, an externalvoltage can be distributed more broadly. This is especially important incase of FCL of resistive types.

Also within the scope of the present invention is a method for producinga superconducting device of an inventive fault current limiter asdescribed above, characterized in that each superconducting element isexposed to a voltage applied transversally across the intermediate layerso current breakthroughs through the intermediate (insulating) layer areinduced, wherein the voltage exposure is carried on until all lowresistance bridges through the insulating layer are burnt out. Thesuperconducting device can be used as resp. within a fault currentlimiter, which is cost efficient in manufacturing, offers short recoverytimes, and can handle high external voltages. By means of the burningout procedure, the insulating performance of the intermediate layer canbe significantly increased for the later practical use. Note that thelatter step can be performed before or after the superconductingelements have been electrically connected.

In a preferred variant of the inventive method, the voltage is appliedas a voltage ramp with a voltage gradually increasing over time, inparticular wherein the voltage increases to its maximum value over atime interval of between 0.3 s and 15 s. In this way, the low resistancebridges are burned out in sequence, i.e. not simultaneously as an“explosion”. This processing is better to control.

Preferably, the processing step in which the superconducting element(resp. its intermediate layer) is exposed to the voltage is performedprior to a processing step in which an electric contact between thesuperconducting film and the electrically conducting substrate throughthe intermediate (insulating) layer of said superconducting element isformed. Otherwise, the electrical contact may be damaged by the voltageexposure.

Further advantages can be extracted from the description and theenclosed drawing. The features mentioned above and below can be used inaccordance with the invention either individually or collectively in anycombination. The embodiments mentioned are not to be understood asexhaustive enumeration but rather have exemplary character for thedescription of the invention.

The invention is shown in the drawing.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 shows a schematic cross sectional view of a superconductingdevice of a fault current limiter for general information, with directelectrical connection of the superconducting films with a continuouscontacting layer;

FIG. 2 shows a schematic cross-sectional view of a superconductingdevice of an inventive fault current limiter, with direct electricalconnection of the superconducting films with local contacting layers atthe interfaces;

FIG. 3 a shows a schematic cross-sectional view of a superconductingdevice of a fault current limiter for general information, with directelectrical connection of the superconducting films, with extended freesurface areas of the superconducting films;

FIG. 3 b shows a schematic plan view of the superconducting device ofFIG. 3 a;

FIG. 4 shows a schematic cross-sectional view of a superconductingdevice of a fault current limiter for general information, withelectrical connection of superconducting films by means of bridgeelements;

FIG. 5 shows a schematic plan view of a superconducting device for afault current limiter for general information, with 90° corners betweensuperconducting elements and bridge elements;

FIG. 6 shows a schematic cross-sectional view of a ring-shapedsuperconducting device for a fault current limiter for generalinformation, with four superconducting elements directly connected;

FIG. 7 a shows a schematic cross-sectional view of a ring-shapedsuperconducting device of a fault current limiter for generalinformation, with two superconducting elements directly connected,

FIG. 7 b shows a schematic cross-sectional view of a ring shapedsuperconducting device of a fault current limiter for generalinformation, with two superconducting elements connected via bridgeelements.

DESCRIPTION OF THE PREFERRED EMBODIMENT

In the figures, superconducting devices for use in a fault currentlimiter are described, with FIG. 2 illustrating the invention whereininsulating layers have central electrical contacts.

FIG. 1 shows a superconducting device 1, with a sequence of threesuperconducting elements 2 a, 2 b, 2 c shown in FIG. 1. Eachsuperconducting element 2 a, 2 b, 2 c comprises an electricallyconducting substrate 3 a, 3 b, 3 c, which is preferably made of a sheetmetal tape, an intermediate layer, i.e. insulating (dielectric) layer 4a, 4 b, 4 c, and a superconducting film 5 a, 5 b, 5 c, preferably a HTSsuperconducting film, and most preferably an YBa₂C_(u3)O_(7-x) film. Thesubstrate thickness TH is typically about 5 μm to 100 μm.

In a concrete example, the substrates 3 a, 3 b, 3 c are based on CrNistainless steel, 0.1 mm thick, 10 mm wide and 200 mm long. Moregenerally, the length of the superconducting elements (measured in thedirection of current flow) may vary from millimeter to several metersize; in our tests, from 20 to 2000 mm have been used, depending on thevoltage drop per unit length in quenched elements (voltage drops from0.3 to 4 V/cm were considered in the example), and on thickness andquality of the intermediate (insulation) layer. The intermediate layercomprising yttria stabilized zirconia layer is from 1 to 5 μm thick inthe given example. The thickness of the YBa₂C_(u3)O_(7-x) film is 1.2μm. The superconducting elements yielded a critical current of 320 A at−196° C.

The superconducting film 5 a of the left superconducting element 2 a iselectrically connected to the superconducting film 5 b ofsuperconducting element 2 b, which is the next superconducting elementin the series connection of the superconducting elements 2 a-2 c of thesuperconducting device 1. The superconducting film 5 b ofsuperconducting element 2 a, in turn, is electrically connected to thesuperconducting film 5 c of the superconducting element 2 c, which isagain the next superconducting element connected in series. Theelectrical connection is established by means of a continuous connectinglayer 6 deposited on top of the central superconducting element 2 b. Theconnection layer typically comprises a multilayer structure which inturn comprises transient sub-layers and a central layer.

The sub-layers are formed on the surface of the superconducting filmalready coated with a very thin (0.07 μm) protection layer of a preciousmetal as Ag or Au. In the present particular example, these sub-layerscomprise also a second layer of Cu, 1 μm thick, deposited via galvanicplating. The sub-layers are provided only within the surface areadevoted for the connection, i.e. the rest surface of the superconductingelements are kept free from any sub-fraction of the connection layer.

The central layer is provided via solder comprising one or more metalsfrom group of In, Sn, Zn, In, Cd, Bi or their combinations. The meltingtemperature of the solder was from 100 to 200° C. The thickness of thecentral layer in the considered example may vary between 2 and 30 μm.The entire interfacial resistance of the connection layer provided inthis example is below 5×10⁻⁷ Ohms×cm² measured at −196° C., i.e. atboiling temperature of liquid nitrogen.

The superconducting elements 2 a, 2 c are aligned in parallel, and thesuperconducting element 2 b is oriented opposite thereto, such that itssuperconducting film 5 b faces the superconducting films 5 a, 5 c.Between the superconducting elements 2 a and 2 c, there is a gap 7,which means that also the electrically conductive substrates 3 a and 3 bare electrically insulated from each other. Accordingly, here theelectrically conducting substrates 3 a, 3 b, 3 c of all superconductingelements 2 a, 2 b, 2 c of the sequence of the superconducting device 1are electrically insulated against each other, what is generallypreferred within the invention.

The gap 7 may be also filled with additional insulator (as e.g. epoxyresin or Teflon) in order to confine spreading of the metallic solder ofthe connection layer within the gap and thus to avoid possibility ofshort-circuiting of the electrically conducting substrates 3 a and 3 c.

Moreover, in order to achieve an improved degree of insulation betweenthe substrate and the superconducting film, each superconducting elementis pre-treated via exposing to a voltage applied transversally acrossthe intermediate layer so current breakthroughs through the electricallyinsulating layer are induced, wherein the voltage exposure is carried onuntil all low resistance bridges through the insulating layer are burntout. A dc voltage that may vary from 10 to 200V is applied between thesuperconducting film and the electrically conducting substrate. Thevoltage is linearly ramped in a way that the maximal voltage is reachedwithin 1-5 seconds; the maximal voltage value is determined prior to thetreatment as about 80% of the breakthrough voltage measured in anelectrically insulating layer of the same type but with homogeneousstructure, i.e. without low resistance bridges. All procedures regardingsuch pre-treatments are preferably performed at room temperatureconditions. By cooling down of the superconducting elements the maximalbreakthrough voltage typically grows.

The superconducting device is, during operation, cooled e.g. with liquidnitrogen (LN2), preferably from both sides (bottom and top), so thateach superconducting element is cooled directly from the side of thesuperconducting film, and through the substrate (the cooling means arenot shown).

In the further superconducting devices revealed in the following FIG. 2through FIG. 7, corresponding features and procedures, in particularwith respect to voltage pre-treatment, cooling means, and insulating gapfillings, may be applied unless otherwise described.

FIG. 2 shows a superconducting device 21 similar to the superconductingdevice shown in FIG. 1. Here, the superconducting films 5 a, 5 b areconnected by a connecting layer part 6 a, and the superconducting films5 b, 5 c are connected connecting layer part 6 b. The connecting layerparts 6 a, 6 b are separate and therefore electrically insulated fromeach other, and are made of a normally conducting metal, in particulargold or silver. The connecting layer parts 6 a, 6 b, accordingly, extendonly in the overlap regions 22 a, 22 b of the superconducting elements 2a, 2 b, 2 c. The surface area parts 23 a, 23 b, 23 c, which are freefrom jointing means (here free from electrically conductive connectionlayer parts) help to increase the resistance of the superconductingdevice 21 under the fault current. This follows from the fact that thequench forms preferably at the portions of the superconducting elementswhich are not “bypassed” by another superconducting element or bridgeelement.

In particular, in the superconducting device 21 of FIG. 2, theinsulating layers 4 a, 4 b, 4 c have central electric contact 24 a, 24b, 24 c, to limit the voltage drop across the insulating layers 4 a-4 c.

It is noted that on top of the superconducting films 5 a-5 c, there maybe deposited a very thin cover layer (or protection layer) of aconducting metal, in particular noble metal such as silver or gold.However, the thickness of this cover layer should be small enough sothat no significant current bypass with respect to the underlyingsuperconducting film is established. Preferably, though, no cover layeris used.

FIG. 3 a and FIG. 3 b show a superconducting device 31, with a sequenceof four superconducting elements 2 a-2 d electrically connected inseries, in cross-section (FIG. 3 a) and top view (FIG. 3 b). Thesuperconducting elements 2 a-2 d have an alternating orientation, withadjacent superconducting elements with their superconducting films 5 a-5d facing each other (note that the insulating layers are not shown here,for simplification). In the top view of FIG. 3 b, the overlap regions 22a, 22 b, 22 c can be well recognized. They make about 20% of the overallsuperconducting films' surface, with the other 80% belonging to freesurface parts 23 a-23 d.

FIG. 4 shows an alternative design for a superconducting device 41, hereshowing a sequence of three superconducting elements 2 a, 2 b, 2 c, withsuperconducting films 5 a, 5 b, 5 c each (again, the intermediateinsulating layers are not shown for simplicity). The superconductingelements 2 a-2 c are all oriented identically with their superconductingfilms facing to the same side (here: top side), and are separated bygaps 45 a, 45 b.

The superconducting films 5 a-5 c are pair-wise electrically connectedin series by bridge elements 42 a, 42 b, each comprising a dielectric(electrically insulating) substrate 43 a, 43 b, and a superconductinglayer 44 a, 44 b, preferably a high temperature superconducting (HTS)film layer. The bridge elements 42 a, 42 b overlap with thesuperconducting elements (resp. superconducting films 5 a-5 c) theyconnect, compare overlap regions 46. In this particular example, thedielectric substrates 43 a, 43 b are made of eitheryttria-stabilized-zirconia ceramics or sapphire (single crystallineAl₂O₃).

FIG. 5 shows a top view of a superconducting device 51, to be used in afault current limiter. There is a sequence of six straightsuperconducting elements 2 a-2 f oriented in parallel, all with theirsuperconducting films face up. By means of five bridge elements 42 a-42e attached on top, the superconducting elements 2 a-2 f (resp. theirsuperconducting films) are electrically connected in series. The brideelements 42 a-42 e are oriented perpendicular to the superconductingelements 2 a-2 f in order to make the superconductive device 51 morecompact. At the beginning and the end of the superconducting device 51(resp. the sequence of superconducting elements 2 a-2 f), there are twometal pads 52 a, 52 b, in particular Cu pads, for external joints, whichare galvanically deposited on the superconducting elements 2 a and 2 f.

FIG. 1 through FIG. 5 have shown linear sequences of superconductingelements. FIG. 5 through FIG. 6 b show ring shaped superconductingdevices, in particular for use in inductive (transformator-based) faultcurrent limiters.

FIG. 6 shows a superconducting device 61 with ring shaped arrangedsuperconducting elements 2 a, 2 b, 2 c, 2 d. Each has an electricallyconducting substrate 3 a-3 d, and intermediate insulating layer 4 a-4 d,and a superconducting film 5 a-5 d. Every superconducting element 2 a-2d is directly electrically connected to its previous and its followingadjacent superconducting element 2 a-2 d, with the electrical connectionestablished only between the superconducting films 5 a-5 d, but notbetween the electrically conducting substrates 3 a-3 d. The electricallyconducting substrates 3 a-3 d of the ring are all electrically insulatedfrom each other. The superconducting elements 2 a-2 d are generally bentas a circular arc.

FIG. 7 a shows a superconducting device 71 for a fault current limitersimilar to the one shown in FIG. 5, but comprising only superconductingelements 2 a, 2 b, with direct electrical connection of theirsuperconducting films 5 a, 5 b. Note that the intermediate insulatinglayers are not shown for simplicity.

FIG. 7 b shows a superconducting device 72, comprising twosuperconducting elements 2 a, 2 b, with their superconducting films 5 a,5 b electrically connected via bridge elements 42 a, 42 b. Again, theintermediate insulating layers are not shown for simplicity.

1. A fault current limiter comprising: a superconducting device having asequence of superconducting elements, each superconducting elementcomprising an electrically conducting substrate, a superconducting filmand an electrically insulating intermediate layer disposed between saidsubstrate and said superconducting film, superconducting films ofadjacent superconducting elements of said sequence being electricallyconnected, wherein said electrically conducting substrate of eachsuperconducting element of said sequence is electrically insulated fromeach electrically conducting substrate of adjacent superconductingelements within said sequence whose superconducting films areelectrically connected in series with said superconducting film of saidsuperconducting element, at least one of said superconducting elementscomprising an electric contact between a respective said superconductingfilm and a respective said electrically conducting substrate through arespective said insulating layer, wherein said electrical contact issubstantially located in a middle between regions where saidsuperconducting element is electrically connected to a previous and to asubsequent superconducting element.
 2. The fault current limiter ofclaim 1, where superconducting films of adjacent superconductingelements of said sequence are electrically connected in series.
 3. Thefault current limiter of claim 1, wherein superconducting films of atleast some adjacent superconducting elements are directly electricallyconnected.
 4. The fault current limiter of claim 3, wherein directlyelectrically connected adjacent superconducting elements are orientedwith superconducting films thereof facing each other and are displacedwith respect to each other, such that adjacent superconducting elementspartially overlap in an overlap region, superconducting films ofoverlapping superconducting elements being electrically connected insaid overlap region.
 5. The fault current limiter of claim 3, whereinsuperconducting films of directly electrically connected, adjacentsuperconducting elements are electrically connected through a layer ofnormally conducting metal.
 6. The fault current limiter of claim 1,wherein superconducting films of at least some adjacent superconductingelements are electrically connected by means of a bridge element, saidbridge element comprising a superconducting section, whereinsuperconducting films of adjacent superconducting elements electricallyconnected by means of said bridge element are electrically connected tosaid superconducting section.
 7. The fault current limiter of claim 6,wherein adjacent superconducting elements electrically connected bymeans of said bridge element are oriented with superconducting filmsthereof facing in a same direction and are arranged next to each other,with a gap being formed between two adjacent superconducting elements,wherein said bridge element establishes an electrical connection, acrosssaid gap, between superconducting films of adjacent superconductingelements.
 8. The fault current limiter of claim 6, wherein said bridgeelement comprises a dielectric substrate, and said superconductingsection is a superconducting layer covering said dielectric substrate,wherein said superconducting layer of said bridge element facessuperconducting films of adjacent superconducting elements electricallyconnected by means of that bridge element, said bridge elementoverlapping or partially overlapping with both adjacent superconductingelements electrically connected by means of said bridge element.
 9. Thefault current limiter of claim 1, wherein said superconducting elementsare connected in a ring shaped fashion.
 10. The fault current limiter ofclaim 1, wherein said superconducting elements are connected in a linearsequence.
 11. The fault current limiter of claim 1, wherein saidsequence of superconducting elements comprises at least threesuperconducting elements.
 12. A method for producing the superconductingdevice of the fault current limiter of claim 1, the method comprisingthe steps of: a) exposing each superconducting element to a voltageapplied transversally across the electrically insulating layer to inducecurrent breakthroughs through the insulating layer; and b) carrying outvoltage exposure until all low resistance bridges through the insulatinglayer are burnt out.
 13. The method of claim 12, wherein the voltage isapplied as a voltage ramp with a voltage gradually increasing over time.14. The method of claim 13, wherein the voltage increases to a maximumvalue over a time interval of between 0.3 s and 15 s.